1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory using carbon-related films as variable resistance layers and a manufacturing method thereof.
2. Description of the Related Art
Recently, demand for non volatile memory devices with small size and large storage capacity has grown. Several ideas for new memory devices have been proposed to overcome the limit of conventional memory devices in terms of the storage capacity.
Among them, a nonvolatile semiconductor memory using a variable resistance material having different resistive states, low and high resistance states, has much attention as is disclosed in document 1 (IEEE Trans. Nanotechnology 1, 39(2002)) and document 2 (Technical Digest, IEDM03 pp. 763-766).
The memory of this type generally includes a variable resistance layer and electrodes disposed to sandwich the variable resistance layer. The variable resistance layer can take two or more different electrical resistance states and the resistance state is changed by applying a voltage pulse between the electrodes, applying a preset threshold voltage or passing a threshold current. Therefore, data can be recorded by changing the resistance state of the variable resistance layer and setting a difference in the resistance in correspondence to the data. Further, data has a characteristic that it can be read in a nondestructive fashion.
As a material of a variable resistance type nonvolatile semiconductor memory developed at present, a metal oxide, for example, a multi-component oxide such as a nickel oxide (NiO) or strontium-zirconium oxide (SrZrO3) is often used. However, the physical and electrical characteristics of the metal oxide are unstable and less reproducible since it is generally difficult to form a metal oxide film with well-controlled composition and crystal structures.
On the other hand, studies on manufacturing methods and materials as candidates for the variable resistance-layer have been carried out for carbon related materials containing carbon nanotubes or fullerene. The above materials are each formed of a single carbon element, and therefore, advantages with the materials are that the composition can be relatively easily controlled and the process thereof can be easily stabilized. However, in a nonvolatile semiconductor memory using the carbon related material as a variable resistance layer, since the adhesion of the variable resistance layer on the bottom electrode is not good enough, there occurs a problem that the variable resistance layer may be separated or peeled off from the bottom electrode due to the film stress thereof.